The present invention relates to a method of manufacturing a movable contact member to be used in equipment such as electromagnetic switches, which member must have great fatigue strength and low hardness. This contact member is also required to be capable of bearing a large current overload, light in weight, chemically stable, capable of good bonding and adequately elastic.
Known techniques to satisfy such requirements include (1) using special alloys, (2) using copper alloys (inclusive of their casting and heat treatment), (3) forming the member in a rather complicated shape so as to provide stiffening, and (4) forming the member by bonding different kinds of alloys with each other. Of those known techniques, the one using a movable contact member comprising a flat spring of age-hardenable copper alloy, such as beryllium-copper-25 alloy (Be Cu 25), with a contact tip bonded to it, has been particularly widely utilized because it results in a simple construction, high electrical conductivity and high mechanical strength.
With that method, however, it is necessary to perform a batch process, age-hardening treatment in a furnace (e.g. at 315.degree. C. for 3 hours) after the step of bonding the contact tip to the flat spring with silver solder. Consequently, some deformation of the product can be caused by heat, so that an additional process (i.e. flattening) is required to correct its shape. Also, this known technique of manufacturing the movable contact member has a disadvantage in that a solid oxide film is formed on the surface of the material in a step usually referred to as "solution treatment". Such an oxide film causes inadequate bonding during the step of soldering the contact tip with that material. Consequently, it is necessary to remove that oxide film by pickling the material before the bonding step, which complicates the manufacturing process.
A further disadvantage of this known technique of manufacturing the movable contact member is that anti-corrosive surface treatment cannot take place before the age-hardening treatment because the heating during the age-hardening treatment would remove the anti-corrosive coating produced by that surface treatment. Since the anti-corrosive surface treatment cannot be done until late in the manufacturing process, a chemical change can occur on the material surface in the earlier stages of the manufacturing process. Still another disadvantage of this known contact member appears during its operation. In particular, its preparation with the age-hardening treatment (e.g. 315.degree. C., 3 hours) results in its structure being excessively age-hardened. This can cause some change in its dimensions during operation if a large current overload occurs in the contact member. Due to the current its temperature can rise over that of the temperature in the age-hardening treatment such that there is a change of its dimensions that can result in faulty performance.
The contact tip used in the contact member is usually made of an expensive material such as silver alloy. It is therefore required that the consumption of the material of the contact tip be minimized in actual operation, in order to obtain a low-cost product. Most of the consumption of the contact tip material occurs due to arcs that appear when contact members part from each other. Such arcs, however, are produced not only upon the breaking of the contact, but also due to bouncing motions of the movable contact member upon closing. If heavy bouncing motions occur, they can not only cause a reduction in the life of the contact members, they can even cause a worst phenomenon, such as the sticking of the contacts together due to melting. It is therefore necessary to minimize those bouncing motions as far as possible. In case the movable contact member is coupled to a movable contact member holder by the force of a spring, the duration t of the bouncing motions is given by ##EQU1## where K is a constant (determined by ratings of the contact, such as the rated voltage and the weight of the movable part), and .epsilon..sub.s is a mechanical constant of the movable contact member relating to its recoil capability. In order to reduce the consumption of the contact tip material, it is desirable to shorten the duration t, that is, to reduce the value of .epsilon..sub.s. Such a reduction in .epsilon..sub.s can be obtained by reducing the hardness of the movable contact member material. But in the case of the above-described and known contact member, reducing its hardness causes a disadvantage, in that its fatique strength drops.